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THE PLANT CELL, Vol 9, Issue 8 1481-1493, Copyright © 1997 by American Society of Plant Biologists
Role of the Proline Knot Motif in Oleosin Endoplasmic Reticulum Topology and Oil Body Targeting
B. M. Abell, L. A. Holbrook, M. Abenes, D. J. Murphy, M. J. Hills and M. M. Moloney
Department of Biological Sciences, University of Calgary, 2500 University Drive Northwest, Calgary, Alberta T2N 1N4, Canada
An Arabidopsis oleosin was used as a model to study oleosin topology and
targeting to oil bodies. Oleosin mRNA was in vitro translated with canine
microsomes in a range of truncated forms. This allowed proteinase K mapping
of the membrane topology. Oleosin maintains a conformation with a
membrane-integrated hydrophobic domain flanked by N- and C-terminal domains
located on the outer microsome surface. This is a unique membrane topology
on the endoplasmic reticulum (ER). Three universally conserved proline
residues within the "proline knot" motif of the oleosin hydrophobic domain
were substituted by leucine residues. After in vitro translation, only
minor differences in proteinase K protection could be observed. These
differences were not apparent in soybean microsomes. No significant
difference in incorporation efficiency on the ER was observed between the
two oleosin forms. However, as an oleosin-[beta]-glucuronidase
translational fusion, the proline knot variant failed to target to oil
bodies in both transient embryo expression and in stably transformed seeds.
Fractionation of transgenic embryos expressing oleosin-[beta]-glucuronidase
fusions showed that the proline knot variant accumulated in the ER to
similar levels compared with the native form. Therefore, the proline knot
motif is not important for ER integration and the determination of topology
but is required for oil body targeting. The loss of the proline knot
results in an intrinsic instability in the oleosin polypeptide during
trafficking.
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